The present invention concerns a method of automatically controlling exposure when printing from film which is exposed under controlled lighting conditions, such as at a portrait studio. The prevailing lighting conditions are determined by photographing a standard subject, in the form of a gray-scale board with at least one field of gray, that is illuminated by the studio lighting.
The German Patent No. 2,840,287 and its corresponding U.S. Pat. No. 4,279,502 disclose a method of determining the levels of the three primary colors of light involved in printing color photographs, whereby the individual frames of a whole strip of film are scanned for each color region by region and differences between the results are established and assigned to the mean density of each point. The functional relationship between the color-density differences and the mean density is then plotted in the form of a curve called the curve of color-density difference. The curve makes it possible to control the colors in the print in terms of the mean density of the original film and leads to very satisfactory results even in the event of color casts. It is of course difficult to determine what points to plot the curve from because points with color dominance or lighting-dictated casts must not be included. The German Patent No. 2,803,381 and its corresponding U.S. Pat. No. 4,274,732 disclose eliminating these drawbacks by exposing an unexposed area of the strip of undeveloped film to a gray-scale image to serve as a color standard once the film has been developed. The gray scale acts as a density-difference curve for the regions without color.
There are, however, several drawbacks to this approach. First, it requires extra exposure. There is very little area left over for the gray scale on ordinary strips of film. Further, the gray scale can be washed out by highly exposed frames. The exposed film may be left in the heat before the gray scale is added. Finally, overall casts can also lead to discoloration. These problems must particularly be avoided in films deriving from portrait studios, where the lighting conditions are precisely known and where the prints must be especially high in quality.